Conventional electric power systems were designed to move central station alternating current (AC) power, via high-voltage transmission lines and lower voltage distribution lines, to households and businesses that used the power in incandescent lights, AC motors, and other AC equipment. Today's electronic devices (such as computers, florescent lights, variable speed drives, and many other household and business appliances and equipment) need direct current (DC) input. However, all of these DC devices require conversion of the building's AC power into DC for use, and that conversion typically uses inefficient rectifiers. Moreover, DC power generated by distributed renewable power sources (such as rooftop solar) must be converted to AC to tie into the building's electric system, and must later be re-converted to DC for many end uses. These AC-DC conversions (or DC-AC-DC in the case of rooftop solar) result in substantial energy losses.
One possible solution is a DC microgrid, which is a DC grid within a building (or serving several buildings) that minimizes or eliminates these conversion losses entirely. In the DC microgrid system, AC power is converted to DC when entering the DC grid using a high-efficiency rectifier, which then distributes the power directly to DC equipment served by the DC grid. On average, such systems reduce AC to DC conversion losses from an average loss of about 10% down to 5%. In addition, roof top photovoltaic (PV) and other distributed DC generation can be fed directly to DC equipment, via the DC microgrid, without the double conversion loss (DC to AC to DC), which would be required if the DC generation output was fed into an AC system.
The major advantages of DC grids are that efficiency can be improved by centralizing part of the power drive train. For DC grids, rectification of AC power and power factor correction can be provided in a single high-power device. A further advantage is that by directly injecting the DC power from PV installations an unnecessary double conversion to and from AC can be dispensed with. This increases the effectiveness of PV installations significantly. A still further advantage is the reduced current stress of power cables since the DC voltage can be selected to be higher than the root mean square (RMS) value of a sinusoidal mains. The DC voltage is typically the peak voltage of the maximum AC mains voltage. Also there are no copper losses associated with reactive power in a DC grid, since there is no reactive power. Finally, partitioning the power in this way causes a large reduction in amount and costs of hardware.
It is much more effective to have one large rectifier and grid controller and very simple load driver (e.g., light emitting diode (LED) driver), than to have a large number of full fledged AC mains drivers, each needing mains filters, a rectifier and a PFC boost module.
Another consequence of the DC grid architecture is that fine grained control can be provided over the grid voltage. This is distinctly different from AC mains where the sinusoidal mains voltage has a varying amplitude and mains current harmonic distortions depending on the load conditions.
Conventional load control approaches (such as 0-10V, Digital Addressable Lighting Interface (DALI), Digital Multiplex (DMX), KNX, etc.) rely on a separate control cable and can also be used with DC lighting. Also powerline communication as described in the IEEE specification 1901 “IEEE Standard for Broadband over Power Line Networks” can be used. However such control solutions are usually quite complex and require additional hardware installations.